Decarbonization of raw metallic filaments for incandescent electric lamps.



- UNITED STATES PATENT OFFICE. ANTON LEDEBER, OF VIENNA, AUSTBIA-HUNGATB:Y,.ASSIGNOB T WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

DEGABBONIZATION on RAW METALLIC FILAMEN'IS FOR mcmnscnnr mnc'rmc LAMPS.

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Specification of Letters I'atent. Application filed May 29, 1909. Serial? No. 499,072.

Patented ten-25', 1913.

7 To all whom it mag concern;

Be it known that I, ANTON LEDERER, a subject of the Emperor of Austria-Hungary, and a resident of Vienna, Austria-Hungary, have invented a new and useful Improvement in the Decarbonization of Raw Metallic Filaments for Incandescent Electric Lamps, of which the following is a specifi cation.

In the manufacture of metal filaments for incandescent electric lamps, it is a verycommon practice to work up powdered refrac f tory metals with a carbonaceous binding material intq a squirtable mass. From this mass filaments are formed, which after being dried, are heated in the absence of oxidizing gases until a so-called raw filament containing the refractory metal and free or combined carbon is obtained. The term raw filament is used throughout the specification and claims in this connection. In order to convert such raw filaments into filaments consisting substantially of pure metal, it is necessary to submit them to a decarbonizing process. A well-known process for removing carbon, is that originally Iproposed by Auer von Welsbach for decar onizing raw osmium filaments in which the raw filaments are heated by means of an electric current in the presence of oxidizing gases, steam for instance, together with a large excess of hydrogen. By this means practically the whole of the carbon is removed in the form of carbon monoxid or dioxid. It is necessary in this process to use a large excess of hydrogen in order to prevent the metal being oxidized as well as the carbon. Attempts have also been made to effect the removal of carbon by passing an electric current through the filaments in an atmosphere containing sulfur-compounds such as sulfureted hydrogen. Nitrogen is also effective to a certain extent, but in all these processes, care must be taken that reducing gases are also present.

All the processes above described are subject to the drawback that it is necessary to treat each filament individually. The raw filament, consisting of metal and carbon, must be connected to the terminals and placed in vessels from which the air is then removed and replaced by the desired atmosphere in which the decarbonization is to take place. The actual decarbonization process is then carried out by heating the filament to white heat by means of an elec- Ztrlc cugrent and in the case of tungsten filaments, 1t occupies a period of from two to three minutes. It is true that several filaments may be decarbonized in this way simultaneously, but the number of filaments which-can be so treated .is limited by difiiculties of manipulation and it is not practlcable to treat morethan about ten filaments simultaneously in the same vessel. Difliculty is experienced in making the apparatus airtight because it must open and close easily and w thout occupying too much time. The difliculties naturally increase with the size of the apparatus. Moreover the manipulation of individual filaments is always attended by considerable danger of breakage. Another disadvantage of the 'process is that the entire heat supplied to the filaments cannot be utilized for decarbonizing because a great part of the heatis conducted away by the gaseous atmosphere, particularly by the hydrogen.

The process forming the subject of my present invention overcomes the drawbacks enumeratedabove; and permitsa considerable simplification of the process'so that the cost of manufacture i's-very largely reduced.

My process makes it ossible to decarbonize several thousand nlaments simultaneously without it being necessary to pass an electric current through each individual filament. In accordance with my invention, I place a large number of raw .iilaments, either directly after the squirting operation or after the squirted filaments have been subjected to dry distillation in a suitable vessel which can be hermetically sealed. This vessel is then exhausted by means of a good vacuum pump and the filaments heated for a sufiicient length of time at a comparatively high temperature. The degree of exhaustion must be such that thechemical action of the residual gasesis of no account. I have found, from experiment, that raw filaments consisting of very finely powdered tungsten and carbon are not attacked when the vessel has been exhausted until the pressure registers 0.2 to 0.5 mm. (absolute) of mercury, even when the temperature is raised to 1500 C. In spite of this fact, however, it is desirable to use as high a degree of exhaustion as possible.

In order to illustrate my invention, I append some observations which I have made when manufacturing tungsten filaments in accordance with the invention. Theraw tungsten filaments containing about 1.5% of carbon were placed in a hermetically sealed vessel which was well exhausted and then gradually heated to about 1200 C. Within a period of one hour the temperature of the filaments was raised from ordinary temperature to about 800 C. and the percentage of carbon was found to have decreased to 1.23%. When the filaments had been heated for a further period of one hour, the temperature being raised to 900 C. the percentage of carbon had diminished to 0.57%. The heating was continued at 1000 C. for 75 minutes and at the end of this 'period, the filaments only contained 0.36% of carbon. After heating the filaments to 1100 C. for one hour, the proportion of carbon sank to 0.06%. A further reduction in the percentage of carbon was effected by continuing the heating to 1200 C. at which temperature the process was stopped, the finished filaments then containing not more than 0.04% of carbon. This figure lies well Within the limits of analysis, because in practice it' is very difficult to keep the filaments free from impurities onaccount of the presence of dust in the atmosphere so that it cannot be said :with certainty whether the 0.04% of carbOn found in the finished filaments is due to the last remaining traces of the carbon from the binding material or whether it is due to impurities accidentally introduced in carrying out the analysis.

In the particular example above described, the degree of exhaustion was such that the pressure measured at the beginning of the experiment amounted to 0.056 mm. of mercury (absolute), but during the heating the vacuum deteriorated until it registered 0.2 mm. mercury (absolute). After-the vessel had been allowed to cool, however, the degree of exhaustion reached its original value, the pressure then being about 0.05 mm. of mercury (absolute).

My researches lead me to believe that the removal of the carbon is effected by means of electrical disintegration although the direct application of electricity does not form part of the process. It is well known that every body when heated to a high state of incandescence, becomes charged with negative electricity. As we have in this particular case two heterogeneous bodies,metal and carbon, constituting the raw filament, each of these will become charged with negative electricity and a mutual repulsion of the electrically charged particles will take place, and the specifically lighter particles, namely the carbon, will be gischarged from the filament with greater orce.

The tungsten filaments obtained by this process are silver gray in color and completely sintered. The length of the filaments diminishes during the process by about 30% and the cross section by about 20%. They have all the properties of pure tungsten filaments so that they can be directly inserted into the bulbs of incandescent lamps.

An electric resistance furnace is found very suitable for the purpose of carrying out my invention but other methods of heating may be adopted.

It should be mentioned that if the metal used in the first instance is not sufiiciently finely powdered, a higher temperature must be used in order that a proper sintering may take place. In the case of metals which vaporize easily, such as osmium, it is desirable to use a lower temperature for a greater length of time. The same is also the case with molybdenum.

I claim as my invention:

1. The process for decarbonizing raw metallic filaments for incandescent electric. lamps which consists in heating the raw filaments in an exhausted vessel, the pressure in which does not exceed 0.5 mm. mercury, until practicallythe whole'of the carbon is eliminated. v

'2...wThe process for decarbonizing raw-metallic filaments for incandescent electric lamps which consists in heating the'raw filaments by means of an external source of heat to a temperature exceeding 800 C. in an exhausted vessel, the pressure in which does not exceed 0.5 mm. of mercury, until practically the whole of the carbon is eliminated.

3. The process for decarbonizing raw tungsten filaments for incandescent electric lamps which consists in placing said filaments in a vessel, exhausting said vessel until the pressuredoes not exceed 0.5 mm. of mercury, heatingsaid vessel to 800 C., gradually raising said temperature to 1200 (l, maintaining the temperature and exhaustion until practically the whole of the carbon is eliminated, and allowing said vessel to cool.

In testimony whereof I have hereunto subscribed my name this 28th day of May 1909.

ANTON LEDERER.

Witnesses:

R. H. HENDERSON, WM. H. CAPEL. 

